This invention relates to an electron lens, equipped with three magnetic pole pieces, capable of eliminating radial and spiral distortions and capable of shortening the focal length thereof.
It has recently been proposed to use as a projector lens a lens equipped with three magnetic pole pieces defining two gaps of opposite excitation. This lens has been found capable of eliminating not only a radial distortion but also a spiral distortion under a certain exciting condition if the magnetic pole pieces are shaped asymmetrical to render an axial magnetic field in the upper gap less intensive than that in the lower gap.
FIG. 1 is a view showing schematically the prior electron lens thus designed. In the figure, two excitation coils 1 and 2, which are connected in series and supplied with the current (I) from a lens power supply 3, are enveloped by a ferromagnetic yoke 4 and non-ferromagnetic spacers 5 and 6. Inside the yoke, the upper pole piece 7, middle pole piece 8 and lower pole piece 9 and their non-ferromagnetic spacers 10 and 11 are installed. The shape of the lens is nearly symmetrical to the center of the middle pole piece. The upper and middle magnetic pole pieces 7 and 8 define an upper gap S1 therebetween, and the middle and lower magnetic pole pieces 8 and 9 define a lower gap S2 therebetween. The length of both gaps S1 and S2 are equal. The bore diameters d2 and d3 of the middle and lower magnetic pole pieces are substantially equal. On the other hand, the diameter d1 of the upper pole piece is 1.5 to 5 times larger than those of the middle and lower pole pieces so that the axial magnetic field in the upper gap is less intensive than that in the lower gap. The turn number (N) of each coil, 1 and 2, is the same and the winding direction of each coil is determined so that polarity of the magnetic field appearing in the upper and lower gaps is opposite to each other and the magnetic field appearing in the upper and lower gaps is generated by the same excitation intensity.
FIG. 2 shows the focal length f.sub.p (mm), radial distortion .DELTA.r/r (%) and spiral distortion .DELTA.S/r (%) of the lens shown in FIG. 1 in relation to the excitation (magnetomotive force) NI (ampere turns). The graph shown is obtained under the condition in which accelerating voltage of the electron beam equals 100 KV. In the event that accelerating voltage does not equal 100 KV, the following conversion equation is established: ##EQU1## where: V*: accelerating voltage (V) of the electron beam corrected "principle of relativity".
NI: value of NI (ampere turns) in the case that accelerating voltage of the electron beam equals V*. PA0 (NI).sub.100 KV : Value of NI (ampere turns) in FIG. 2.
As is noted from FIG. 2, the focal length f.sub.p shows a minimum value (about 5.3 mm) at an excitation value in the vicinity of 2700 AT, and the radial distortion .DELTA.r/r and spiral distortion .DELTA.S/r are eliminated at an excitation NI in the range of 2800 to 2900 AT.
With such a lens, however, since the minimum focal length is 5.3 mm, the magnification Mp of a projector lens is about 72 times for an electron microscope in which the projector lens is spaced from film by a distance L=380 mm. Therefore, high-magnification observation such as 500,000 or 1,000,000 times cannot be made with such a unit, and the projector lens needs to be changed between high and low magnification observations.
Now, it should be considered first why the focal length of a triple-pole piece lens is longer than that of a bi-pole piece lens. The focal length of an ordinary bi-pole piece lens can be expressed by a Liebmann curve. On the other hand, with a triple-pole piece lens, the focal length is minimum under a certain magnetomotive force, and it increases with an increase in the excitation. This phenomenon arises out of the fact that the magnetic field generated in the upper gap S1 serves as a reduction lens.